SCALES OF FISHES. They are divided by Agassiz, whose classification is generally adopted, into the placoid, ga noid, ctenoid, and cycloid forms. Placoid scales (from the Gr. plax a broad plate) lie side by side without overlapping or imbricating. They are often elevated at the center so as to form a strong projecting point. All the cartilaginous fishes, except the stur,geon, have placoid scales. Ga noid scales (from the Gr. ganos, splendor) arc covered with a flue enamel, and generally of a rhomboidal form. and imbricated. The sturgeon and the bony pike (lepidosteus) have scales of this nature, but the finest examples of these scales are found in fossil fishes. , Ctenoid 'scales (from kteis, a comb) are generally of a rounded or oval form, with teeth or projections on their posterior margin. • They are devoid of enamel, and present an imbricated arrangement. The perch and many osse ous fishes possess these scales. Cycloid scales (from the Gr. kyktos, a circle) consist of concentric layers of horn or bone, without spinous margins, and not covered by enamel.
They are soft and flexible, present a variety of linear markings on their upper surface, and usually exhibit an imbricated arrangement. The carp, herring, salmon, etc., possess these scales. In many cases, two kinds of scales occur in the same fish, while in other cases the different species of a single genus exhibit different kinds of scales.
For anatomical details regarding the structure and mode of development of scales, the reader is referred to prof. Iluxley's article " Tegumentary Organs" in the Cyelopccdhn of Anatomy and Physiology, and to prof. Williamson's memoirs in the Philosophical Transactions, 1S49-52. In their chemical composition, the scales of fishes approxi mate to the bones, except that they contain more organic matter. The brilliancy of tint exhibited by many fishes is due apparently,to the phenomena of optical interference, rather than to the presence of coloring manner. Figures of ctenoid and ganoid scales are given in the articles CTENOID FISIIES and GANoto FtsitEs.